In case that foreign substances including bacteria, virus or parasite invade living organisms, immune systems exist in order to exclude said substances. In acquired immune systems, antigen processing by antigen presenting cells such as dendritic cells (DCs) is carried out when the foreign substances invade, and naive Th cells functionally differentiate via interactions of DCs/Th cells into Th1 cells or Th2 cells which play a central role of immune response in vivo. It is believed that immune diseases are developed by one-way deflection of immuno-balance of Th1 cells or Th2 cells in this process.
Specifically, cytokine such as interleukin-4 (IL-4) and interleukin-5 (IL-5) secreted by Th2 cells is secreted in an excess amount within the body of patients with allergic diseases, and a compound inhibiting immune response of Th2 cells may be expected to be a therapeutic agent for allergic diseases. Also, a compound enhancing immune response of Th1 cells such as interferon α-inducing activity may be expected to be a therapeutic or preventive agent for viral diseases, cancers, etc.
In the meantime, it was believed until recently that natural immune system was caused by nonspecific phagocytosis, but it was proved that Toll-like receptor (TLR) exists and principal parts of natural immunity activation are carried out via TLR. Moreover, since TLR recognizes a ligand to induce inflammatory cytokine such as IL-12, TNF, etc. and IL-12 differentiates and induces naive T cell to Th1 cell, a ligand of TLR may be expected to have a function as a Th1/Th2 differentiation controlling agent and to be useful for treatment or prevention of immune diseases. Actually, it is known that Th2 cell predominates in patients with asthma, atopic dermatitis, etc., and asthma-targeted clinical trials are carried out for DNA (CpG DNA) derived from microorganism, which is TLR9 agonist. Additionally, it is known that TLR7/8 agonist imidazoquinoline derivative (see Patent Document 1) also shows producing inhibitory activity of Th2 cytokine, interleukin-4 (IL-4) and interleukin-5 (IL-5), and it is actually effective for allergic diseases in animal models.
Meanwhile, compounds described in, for example, Patent Documents 2 to 4 are known as compounds with adenine skeletons which are effective for immune diseases such as viral diseases and allergic diseases.    Patent Document 1: U.S. Pat. No. 4,689,338    Patent Document 2: WO 98/01448    Patent Document 3: WO 99/28321    Patent Document 4: WO 04/029054